Double-blind, placebo-controlled, randomized multicenter study on the efficacy and safety of Ibandronate during 3 years treatment in patients with postmenopausal osteoporosis using an intermittent injection regimen. The purpose of this three year phase III study is to investigate the therapeutic effect of Ibandronate in the long-term treatment of postmenopausal osteoporosis by evaluating the incidence of new vertebral fractures as the primary endpoint.